[cgiapp] Yet another callback system proposal

[Michael Graham <suppressed>]

Okay, here's a new approach to callbacks that solves the same issues,
but doesn't require a Plugin base class.

Basically, I changed the callback mechanism from object-based to class-
based.  Instead of storing hooks and callbacks in $self->{__HOOKS},
hooks are now stored in the hash %INSTALLED_HOOKS.

However, the each callback is keyed by a particular class; only
callbacks installed by plugins related to the current application are
ever run.

A plugin can create hooks and add callbacks at any time - even before
the $self object has been initialized.  A good place to do this is in
the import subroutine:

  sub import {
      CGI::Application->add_callback('init', 'my_setup', 'FIRST', scalar(caller));
      goto &Exporter::import;
  }

When CGI::Application installs the callback, it records the class of the
module that used the plugin (hence the "scalar(caller)" parameter).

When the callbacks for a particular hook are triggered, CGI::Application
runs all callbacks installed by plugins that were used by $self or
by superclasses of $self.

It skips callbacks installed by plugins that are used by modules
unrelated to $self.

In a persistent environment, there might be a lot of applications
in memory at the same time.  For instance:

    CGI::Application
      Other::Project   # uses CGI::Application::Plugin::Baz
         Other::App    # uses CGI::Application::Plugin::Bam

      My::Project      # uses CGI::Application::Plugin::Foo
         My::App       # uses CGI::Application::Plugin::Bar


When My::App runs, only the callbacks installed by the following plugins
will run:

    CGI::Application::Plugin::Foo
    CGI::Application::Plugin::Bar

The other callbacks are skipped.

I have also changed the position scheme.  Instead of limiting the
positions to 'REALLY_FIRST', 'FIRST', 'MIDDLE', 'LAST', and
'REALLY_LAST', you can now specify a numeric position between 0 and 100.

The named positions are still available, but they are aliases for
numeric positions:

    REALLY_FIRST  =>  5
    FIRST         => 15
    MIDDLE        => 50
    DONTCARE      => 50
    LAST          => 85
    REALLY_LAST   => 95

So, the following two calls are equivalent:

    CGI::Application->add_callback('teardown', 'my_fixup', 'LAST',  scalar(caller));
    CGI::Application->add_callback('teardown', 'my_fixup',  85,     scalar(caller));

This gives plugin authors some flexibility in the ordering of their
callbacks.

I've made a tarball available here:

  http://www.occamstoothbrush.com/downloads/perl/CGI-Application-4.0_2-class-callbacks-patch.tgz

The example script is called 'test-class-callbacks.pl'.


Michael



-----------------------------------------------------------------------
Michael Graham <suppressed>

YAPC::NA 2005 Toronto - http://www.yapc.org/America/ - suppressed
-----------------------------------------------------------------------


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